There are more than 100 types of arthritis, but most fall into a handful of major categories. Roughly 21% of American adults have been diagnosed with some form of the condition, making it one of the most common causes of chronic pain and disability. Understanding which type you’re dealing with matters because the causes, symptoms, and treatments differ significantly from one form to the next.
The broadest distinction is between degenerative arthritis, where joint cartilage physically wears down over time, and inflammatory arthritis, where an overactive immune system attacks the joints. A third, smaller category covers arthritis caused by infections or crystal deposits. Here’s what sets each major type apart.
Osteoarthritis: The Wear-and-Tear Type
Osteoarthritis is by far the most common form. It develops when the cartilage cushioning a joint gradually breaks down, exposing the bone underneath. As that normally smooth surface erodes, the joint becomes painful to move and range of motion shrinks. The damage starts with tiny cracks and fraying in the outer layer of cartilage and progresses over years into deeper erosions you can see on imaging.
What drives this breakdown is a chemical imbalance inside the joint. Healthy cartilage constantly rebuilds itself, but in osteoarthritis the enzymes that break down cartilage overwhelm the body’s ability to repair it. Inflammatory signals accelerate the process, triggering further cartilage loss, cell death, and swelling. This is why osteoarthritis isn’t purely “mechanical” wear, even though age, joint injuries, and repetitive use are its biggest risk factors.
Osteoarthritis most commonly affects the knees, hips, hands, and spine. It tends to develop after age 50, though joint injuries earlier in life can set the stage decades before symptoms appear. Pain typically worsens with activity and improves with rest, which helps distinguish it from inflammatory types where stiffness is worst in the morning.
Rheumatoid Arthritis
Rheumatoid arthritis is the most well-known inflammatory type. Instead of cartilage wearing out mechanically, the immune system attacks the tissue lining the joints, called the synovium. White blood cells flood into the joint space, and the inflamed synovial tissue begins to thicken and invade surrounding cartilage and bone. Left unchecked, this process erodes the joint from the inside out.
The immune attack involves multiple players. Certain immune cells activate inflammatory compounds that recruit still more immune cells, creating a self-sustaining cycle of damage. B cells produce autoantibodies, including rheumatoid factor (RF) and anti-CCP antibodies, which are the markers doctors test for in blood work. Having these antibodies, particularly anti-CCP, is associated with more aggressive joint erosion over time.
Rheumatoid arthritis typically starts in the small joints of the hands and feet, often symmetrically (both wrists at once, for example). Morning stiffness lasting longer than 30 minutes is a hallmark. Unlike osteoarthritis, it can also cause fatigue, low-grade fevers, and inflammation in other organs. It often appears between ages 30 and 60 and is two to three times more common in women.
Gout and Crystal Arthritis
Gout is caused by uric acid crystals forming inside a joint, triggering intense inflammation. It’s one of the most acutely painful forms of arthritis. Attacks often strike the base of the big toe, though ankles, knees, and wrists can also be affected. The joint becomes red, hot, swollen, and exquisitely tender, sometimes within hours.
Uric acid is a normal byproduct of breaking down certain foods and body tissues. Problems start when levels climb too high: above roughly 7 mg/dL in men or 6 mg/dL in women. At that point, needle-shaped crystals can form in joint fluid. Any sudden shift in uric acid levels, up or down, can trigger a flare. Common triggers include red meat, seafood, alcohol, dehydration, certain medications like diuretics, surgery, and acute illness. Even starting uric acid-lowering medication can paradoxically set off an attack at first.
A related but less common condition called pseudogout involves calcium crystals rather than uric acid. It tends to affect the knees and wrists and is more common in older adults. Both are confirmed by examining joint fluid under a microscope to identify the crystal type.
Psoriatic Arthritis
Psoriatic arthritis develops in some people who have psoriasis, the skin condition that causes red, scaly patches. Joint symptoms can appear years after the skin disease or, less commonly, before any skin changes are visible. It shares features with rheumatoid arthritis but has its own distinct patterns.
One hallmark is dactylitis, sometimes called “sausage fingers” or “sausage toes,” where an entire digit swells rather than just a single joint. Dactylitis affects the feet more often than the hands (about 65% of cases involve only the feet) and tends to appear asymmetrically. Digits affected by dactylitis show more joint damage on X-rays than unaffected digits, making it a marker of more severe disease. Psoriatic arthritis can also cause inflammation where tendons attach to bone, pitting or separation of the fingernails, and lower back pain if the spine is involved.
Ankylosing Spondylitis
Ankylosing spondylitis primarily targets the spine. Inflammation begins at the joints connecting the spine to the pelvis (the sacroiliac joints) and gradually works its way up the vertebrae. Over time, the inflamed vertebrae can fuse together, a process called ankylosis, which progressively limits back movement. In severe cases, the spine becomes rigid in a fixed, forward-curved position.
A genetic variant called HLA-B27 significantly raises the risk of developing this condition. However, most people who carry HLA-B27 never develop ankylosing spondylitis, and some people with the disease don’t carry the gene at all. It typically appears in the late teens or twenties and is more common in men. Early symptoms include chronic lower back pain and stiffness that improve with movement but worsen with rest, the opposite pattern of a herniated disc or muscle strain. It can also affect the hips, shoulders, and eyes.
Juvenile Idiopathic Arthritis
Arthritis isn’t limited to adults. Juvenile idiopathic arthritis (JIA) is a group of conditions defined by joint inflammation that begins before age 16. “Idiopathic” means the cause isn’t fully understood, though it involves an immune system that attacks joint tissue in much the same way as adult inflammatory arthritis.
JIA is classified into several subtypes based on how many joints are involved and what other symptoms appear:
- Oligoarticular JIA affects four or fewer joints and is the most common subtype.
- Polyarticular JIA affects five or more joints and can be further divided by whether rheumatoid factor is present.
- Systemic JIA causes joint inflammation along with high fevers and rashes.
- Enthesitis-related arthritis involves inflammation where tendons meet bone, often in the legs and spine.
- Psoriatic JIA combines joint inflammation with psoriasis or a family history of it.
Some children outgrow JIA, while others carry it into adulthood. Early treatment is important because uncontrolled inflammation during growth can affect bone development and joint function long-term.
Septic Arthritis
Septic arthritis is a joint infection, most commonly caused by the bacterium Staphylococcus aureus, though viruses and fungi can also be responsible. It usually affects a single joint (the knee is most common) and causes rapid-onset pain, swelling, warmth, and fever. This form is a medical emergency. The infection can damage cartilage and bone quickly, and delayed treatment leads to permanent joint destruction.
Diagnosis requires analyzing fluid drawn from the joint and culturing it to identify the specific organism. People with existing joint disease, joint replacements, weakened immune systems, or recent joint surgery are at higher risk.
How Different Types Are Told Apart
Because many forms of arthritis cause similar symptoms, telling them apart often requires a combination of physical examination, blood tests, imaging, and sometimes joint fluid analysis. Blood tests for rheumatoid factor, anti-CCP antibodies, uric acid levels, and markers of inflammation help narrow the field. Imaging plays a different role depending on the stage and type. X-rays can reveal the bone erosions and joint space narrowing seen in advanced rheumatoid arthritis or osteoarthritis, but they often look normal early on.
Ultrasound has become increasingly useful for detecting inflammation that isn’t obvious on physical exam. It can show thickening of the joint lining, increased blood flow to inflamed tissue, and early bone erosions. MRI offers the most detailed view of soft tissue and bone and is particularly valuable for spotting spinal inflammation in ankylosing spondylitis. For suspected gout or septic arthritis, drawing fluid from the joint is often the definitive test, revealing crystals under a microscope or bacteria in a culture.
The type of arthritis you have shapes everything from which medications work to how the condition progresses. Getting a specific diagnosis, not just a general label of “arthritis,” is the first step toward the right treatment plan.